That the meme frames "not being in shape as something that needs to be excused" is your interpretation. I see it as a response to the prevalence of excuses that are trotted out by people when explaining why they won't take the steps of engaging in diet or exercise in the first place. Obviously if you're not making excuses to begin with it doesn't apply, but many people do so. Whether it is an effective challenger of those excuses or whether it turns people away is up for debate; judging by this thread I'd lean towards the latter.
As for Opiate's examples, they were silly. The basic point that people have different contexts influencing their achievements is valid, but the specifics were not comparable. If you think the barriers to entry and amount of time, effort and even natural predisposition required to obtain a degree from a prestigious university are comparable to those required to engage in a basic physical training program you're either ill-informed or being dishonest. But Opiate's examples fail at an even earlier hurdle: he is comparing outcomes to processes. These memes (by and large) don't contain pictures of Olympic medallists or models or bodybuilders and then ask people why they haven't reached a similar level of achievement, they show people with a large disadvantage making an effort. The (purported) 102 year old on the rowing machine isn't going to win Mr. Olympia, but he is applauded for not letting excuses dissuade him. The woman in the OP isn't saying "I've had kids too so why don't you look as good as me?", she's saying "Despite having children I make time to exercise." Her appearance is simply a visual heuristic for confirming this. It may be an attention grabbing and obnoxious one, but that's all it is.
If you take a look at the fitness thread on this very forum (and many others like it), you'll find that most people don't care if you're fat or weak or skinny or slow, they care if you have a positive attitude and are willing to learn. When people say things like "I don't have 20 hours a week to spend lifting weights" it demonstrates that they are unwilling to put even the slightest effort into researching the topic and thus are "making excuses". It certainly doesn't help that vast swathes of the fitness industry peddle FUD aimed at confusing customers, distorting their body images and hampering their efforts to improve their health in order to prime them for continued consumption, but lashing out any time you're told that you've been sold a lie is a good way to ensure that you'll keep making excuses and that any effort you do make will likely be in vain.
No, she explicitly is saying that: "You're not in shape: what's your excuse now that I've proven one of those excuses can be overcome." Perhaps a person is a single mother with three kids. Is that good enough excuse? The entire enterprise is reductionist of a very complex issue, and as such loses its messaging because it only honestly applies to people very similar in life to her.
The word excuse is saying that being in shape is the optimal lifestyle choice, and you must find reasons for not being in shape. And while I would agree that being in shape is conducive to a healthier life, not everyone agrees it is a better life, and society itself does not make such a thing necessary.
So when you compare this thinking to subjects that objectively improve the quality of life as set by society (school, finance), it further broadens the scope of how condescending the attitude is.
Everyone has things they're good or bad at, and it is not empathetic to assume that people should rise to your standard in whatever niche you excel within. Be it academia or physical capabilities or anything else.
The argument should fall on whether "tough love" is an effective form of motivation when applied to a population. It can certainly work on an individual basis, but I'm of the opinion that society in general deserves the scrutiny and retooling - not the overweight+ demographic.
I don't see the word "fault" in that definition though...you said it was literally in the definition.
Mistake, bad behavior...all "faults". Explicitly.